T. Fehringer
M.S. Gussenhoven;F. Rich; C. Huang
The polar cap boundary has been defined by
a variety of physical conditions: The poleward boundary of discrete
aurora, the onset of polar rain, the termination of ion precipitation,
and plasma convection reversal, to name the more common ones.
Although most theoretical models of the physical processes in
the magnetosphere indicate that these boundaries should coincide,
in fact, they do not. In a recent statistical study, using data
from the DMSP satellites, we found that, on average, the termination
of ion precipitation occurred at latitudes 2 -3 degrees higher
than the plasma convection reversal in the dawn and dusk sectors
of the polar cap-auroral region. However, the variation about
the average was very large, and in many cases the convection reversal
could be at higher latitudes than the ion precipitation boundary.
The October 18-20, 1995 passage of a magnetic cloud affords the
opportunity to better understand what determines the relationship
between the two boundaries since solar wind conditions are near-constant
(speed, density, temperature, magnitude of B, and sector) or slowly
varying (Bz, from large and negative to large and positive) for
the better part of a 24 hour period. We find that even under
slowly varying conditions the relationship between the ion poleward
boundary and the convection reversal is extremely complex, even
though there is an overall trend for both to move to higher latitudes
as Bz turns northward. The correlation of boundary positions
to Bz is far greater, in this case, than to Kp.
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